Photo-lithographic and ablative patterning of two dimensional structures are used in the manufacture of printed circuit boards, multi-chip modules, flat panels, chips, and the like. On a generalized basis, a mask contains an image to be broadcast. This will hereafter be referred to as the broadcast image. Typically, this broadcast image is projected to a workpiece. At the workpiece, the broadcast image produces a result. This result can be an image on photoresist which is later processed. Alternately, and in the case of ablative image processing, the image on the workpiece can be directly produced in the form of ablative patterning. It will be understood that in what follows, the examples of this disclosure are applicable to both types of systems.
Contact printing of workpieces is known. In this case, the broadcast image is in direct contact with the workpiece. The image is directly transferred from image to workpiece with little initial degradation.
Unfortunately, contact between mask and workpiece rapidly degrades the quality of the broadcast image from the mask if numerous prints must be made. This being the case, proximity imaging is utilized.
In proximity printing, a gap is maintained between the mask and the workpiece. This leads to a loss of resolution due to diffraction effects and other optical artifacts.
In Henry I. Smith et al. "Lithography Mask with A Pi-Phase Shifting Attenuator", U. S. Pat. No 4,890,309, issued Dec. 26, 1989, a technique for ameliorating this effects with pi (.pi.) phase shifts on the mask with the broadcast image is shown. Unfortunately, utilizing this technique, diffraction due to the gap between the mask having the image to be broadcast and the workpiece still dominates image sharpness.
Projection printing is well known. Specifically, a projection lens system is utilized. The image to be projected is place at one conjugate of the projection image lens system. The surface onto which the image is to be projected (workpiece) is placed at the corresponding conjugate. Generally, projection printing is satisfactory; it produces good resolution, and the gap between the optic producing the projection and the workpiece is reasonable. However, it is well known that projection systems are expensive. With increasing field, lens system expense correspondingly increases.